Amplifier



P. H. NELSON May 30, 1933.

AMPLIFIER Filed April 4, 1930 ,llllllillll INVENTOR Pal/l li/lelson ATTORNEY Patented May 30, 1933 UNITED STATES L oTRIGcs MANUFAGTURINGCOMPANY, A CORPORATION orrnmtsyrivnnmf ,PAUL H. NELSON, or COLLINGSWOOD, NEW .innsny; AssIeNoR ardwnsrnienon'sn AMPLIFIER Application iiledyApii14, 1980. Serial No. 441,603. 5

My inventioni'relates to amplifying sys tems, and more particularly to systems for use in connection With phonograph pick-up devices, talkingmoving pictures and the like.

In devices of the'typereferr'ed to above, it

has been exceedingly difficult to obtain'pure reproduction of musical tones. Verylittle trouble has been experienced in amplifying speech-modulated currents, as the range of frequencies covered by the voice ismore or less limited and lies Within the most eflicient ortion of the amplifying characteristic of finown commercial apparatus. Musical tones, however, spread over a band of frequencies much broader than that covered by the voice and these frequencies, toward the limits of the audio frequency band, are, consequently, imperfectly amplified, thuspro ducing a certain amount of distortion in the amplifier output. V s

It is an object of my invention to provide an amplifying system capable of producing substantially uniform amplification over a Wide range of frequencies. 1

Another objectof my invention is to provide variable inter-stage coupling means whereby frequency-response curves ofdiflerent desirable characteristics may be obtained.

Another object of my invention is to provide an amplifier for utilizing screen grid tubes and having the characteristics set forth may be altered Without either appreciably aflectingthe other.

In the accompanying drawing, the single of. invention is to profigure a diagram of apparatus and circuits embodying my invention.

[A photo cellcontrolled circuit 1..is :dis

closed as comprising the input supply source;

for the amplifying stages '2 and3 referred to above: Since practlcally any. desirable circuit may be used in combination with my improved amplifier, a detailed descriptionof the photo-cell circuitsis not considereclnecessary. In lieuof the photo-cell-controlled circuit, I might, for'example, impress, across the input circuit of the amplifier, "a voltage from a phonograph pick-up device or a radio set through a suitable impedance matching transformer.

A source of potential 4 comprisesthe on5 ventional A. G. rectifier and filter. circuits for.

supplying" D. C. potentials to thermionic-l tubecircuits, as ell as A. 'C. of suitable potential for heating the cathodes of'the amplif fying tubes. s s I The specific embodiment of my invention is-shown incorporated in .thecoupling ar rangement provided betiveen two. stages. of. push-pull amplification for utilizing screengrid tubesin the first stage. p

The first stage comprises a pair. of push-f pull connected tubes5 and 6 of the screengrid heater type, such as the U. 224typeY tube, the filaments of which. are supplied from the heater coil of the,potential,-supply An inputcoil 9, inductively 'coupled'tothe output llof the photo-cell-controlled circuits, is connected between the grids of the push? pull connected tubes, the tube cathodes 13' and 15-beingelectrically connected, through a grid-bias resistor 17, to a pointon said coil intermediate its extremities, Whichpointis joined, by means of the conductor21,-to the low-potential side of the voltage-supply source] f I Aco ndenser 23 is provided; to by-pass the resistor 17 to alloyv a free" path I for the high-I frequency component of the space-discharge currents. .The anodes fio'f the 'tuhesfare'fcon nected to the high potential side offthevolb age-supply sourcethrol gh a resistance 25 of appronim'ately 100,000 ohms and an; indu'ctance 27 of approximately 4 henries, in series connection, as shown.

The screen grids are tapped off at a lower 5 potential, the resistance 29 providing the re- 250, in push-pull connectiomthe input ci-rcuit comprising an inductor of approximately 5,000 henries joining the grids of the power tubes, a point 37 on the inductor being connected to the electrical center 39 of the filament circuit through a grid-bias resistor 41 and a by-pass condenser 43.

The filament current is supplied from a heating coil 45 of the potential-supply source. The primary of transformer 47' constitutes the output circuit of the power stage, a connection to the high-potential side of the voltage-supply source being made from a point 49 intermediate its extremities.

In using screen grid tubes in the amplifier,

' a very high impedance in the plate circuit is necessary because of the high impedance of the tubes, in order that a satisfactoryoutput may be obtained. The use of inductive impedance was found to be entirely impractical ,as it would necessitate an estimated inductance of 2,000 henries or more to accomplish the desired condition. If such an inductance were utilized, the many" turns of wire required in its construction would cause it to have such large distributed capacitance as to effectively short circuit it for all high frequencies. In addition to this effect, the variation of impedance of the reactor, as the frequency was changed, would cause the tubes to amplify non-uniformly with the varying frequency. One result of this phenomenon would be a loss of the low frequenoies,as the reactor presents a low impedance at the low audio frequencies. Aresistor also was found unsatisfactory. In both cases noted above,

becauseof the high impedance, various stray shunt capacities permitted by-pass' paths for the higher frequencies, with the result that the output-response curve of the amplifier was very low at the high-frequency end.

"Very satisfactory results Were obtained by reducing the Value of resistance from about 250,000 ohms to approximately 100,000 ohms and inserting, in series therewith, an impedance of approximately 4 henries having very low distributed capacitance.

The above values assigned to the'constants of my improved circuit will result in a sub stantially uniform response curve between the limits of 30 cycles and 8,000 cycles, which practically covers the audio-frequency range.

quencies to a greater degree than the others.

" elements of the amplifier variable, the re;

It has been found desirable, at times, to change the characteristics of the response curve in a predetermined manner to compensate for acoustical defects of rooms or auditoriums. .Should a particular auditorium be inefficient in its response to the high frequencies, it would be very desirable to be able to so change the response curve of the amplifier as to amplify the high fre- In my system, this can be accomplished by increasing the value of the reactors 27. A similar effect may be obtained on the low frequencies by so increasing the coupling capacities 31 and 33 that the coupling capacitors form a series resonant circuit with the reactor 35 across the grids of the amplifying stage. Therefore, by making the above sponse curve of the amplifier can be varied to suit the conditions under which it is to be used.

While'I have disclosed my invention in great detail, it is apparent that various changes may be made Without departing from the scope of the invention and I do not desire to be limited to the specific details except in so far as shall be required by the prior art and by the appended claim.

I claim as my invention; 7

An amplifying system comprising a thermionic device of the screen grid type, an output circuit for said thermionic device, said output circuit having an impedance substantially equal to that of said thermionic de vice throughout a wide band of frequencies and comprising a resistor and a reactor hav-' ing low distributed capacity; and an additional thermionic device, said last-mentioned thermionic device having an input circuit 195 associated therewith and coupling means between said output circuit and said input circuit, said input circuit and said coupling means together forming a series circuit tuna able to resonate at the lower audio frequencies. e a

In testimony whereof, I have hereunto subscribed my name this 26th day of March 1930.

' Y 7 PAUL H. NELSON.

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